January 2, 2012

Animals are complex multicelullar eukaryotic organisms. The key word here is complex. There are multicellular prokaryotic organisms like the pond scum Volvox, but what distinguishes plants, animals, and fungi from the protista is the level of organization. Unlike single-celled oranism, animals have body plans. A body plan is the blueprint for the way the body of an organism is laid out. Animal body plans describe the symmetry, body cavity structure, segmentation, and appendages that each animal has. There are two major ways that an animal’s body plan can exhibit symmetry; radial and bilateral. Some organisms like sponges, however, lack symmetry of any kind, but other creatures like sea stars, sea anemones, and jellyfish exhibit radial symmetry. Animal plans that exhibit radial symmetry have no left or right sides.
Animals body plans that exhibit bilateral symmetry have identical left and right symmetrical halves. Most animals with radial symmetry move slowly or not at all, while animals with bilateral symmetry are able to move more rapidly. Many bilaterally symmetrical animals are cephalized, with sensory and nervous tissues in the head. Cephalization is a trend where bilaterally symmetric animal genera tend to exhibit a distinct anterior head that directs the body. Bilateral symmetry permits streamlining, favors the formation of a central nerve center, and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates like fish, amphibians, reptiles, birds, and mammals.
Early animals did not have an enclosed body cavity. These creatures like flatworms are called acoelomate. Their gut is surrounded by a layer of mesenchyme, which is a loose connective tissue that attaches the gut to a layer of muscle. Roundworms are called pseudocoelomates because they have a fluid-filled body cavity called called a pseudocoelom. The pseudocoelom is lined with a layer of cells called a mesoderm, with the internal organs supported only by the hydrostatic pressure of the fluid within the pseudocoelom. The most complex body cavity arrangement belong to the coelomates. Coelomates have a true coelom, a body cavity surrounded entirely by a layer of mesoderm cells called the peritoneum. The internal organs of coelomates are also surrounded by a layer of peritoneum. This layer of connective tissue can act as a shock absorber against the assaults from the outside world and provides additional stability as a hydrostatic skeleton. A hydrostatic skeleton provides support because the liquid in the coelom can’t be compressed, and the volume and shape of the organism remains constant due to hydrostatic pressure.
Some organisms exhibit segmentation of body parts; segmentation is the repetition of similarly-sized-and-shaped body parts, usually in a series. Segmentation improves control of movement, especially if the animal also has multiple appendages. This is where Areiosan life differs from Terroan life; on Earth most organisms have an even number of appendages; insects have six legs, arachnids have eight legs, and most amphibians, reptiles, and mammals are four-legged tetrapods. Due to a curious evolutionary quirk in the genes that code for the development of all animals on Areios, Areiosan creatures have an odd number of limbs. On this world, one would find almost exclusively seven-legged bugs and five-legged quintapod reptiles. The genetics of how organisms develop is important to understanding evolutionary biology and the history of animal life.

On Areios seven-limbed echinoderms like this sea star are commonplace.